1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic system such as a copying machine and a printer.
2. Description of the Related Art
According to a conventional image forming apparatus, when a sheet is nipped by a nip portion of a fixing apparatus and a transfer apparatus and conveyed in this state, if the sheet is pulled from the fixing apparatus, deviation or rubbing of an image is generated in some cases. To prevent the deviation and rubbing of an image, there is a known image forming apparatus having a configuration that a sheet is looped and conveyed in this state upstream of the fixing apparatus. According to this image forming apparatus, passing timing of a sheet is detected using a sensor which is disposed on an intermediate portion of a conveying path, a rotation velocity of a roller of the fixing apparatus is variably controlled according to the detected timing, thereby generating a loop in the sheet at an upstream portion of the fixing apparatus.
However, it is difficult to constantly maintain a conveying velocity of a sheet due to factors such as variation in machining precision of parts constituting the apparatus and thermal expansion, and as a result, it is difficult to constantly maintain a loop amount of a sheet.
Hence, there is a known image forming apparatus having a device which detects a loop amount and maintains the loop amount within a predetermined range based on the detection (Japanese Patent Application Laid-open No. H7-234604). That is, the image forming apparatus body includes a sensor which detects an upper limit and a lower limit of the loop amount formed by a sheet, and control is performed such that when the upper limit of the loop amount is detected, a velocity of a driving portion of the fixing apparatus is increased, and when the lower limit of the loop amount is detected, the velocity of the driving portion of the fixing apparatus is reduced.
To detect the loop amount described in Japanese Patent Application Laid-open No. 7-234604, if a mechanical sensor is disposed between the transfer apparatus and the fixing apparatus and the sensor is brought into contact with a sheet, a problem concerning a conveying operation of the sheet is generated in some cases. When a sheet (thin paper sheet) having a basis weight of 60 g/m2 or less is to be conveyed, since elasticity of the sheet is weak, there are problems that a tip end of the sheet (sheet tip end, hereinafter) collides against the mechanical sensor which is a loop detector and a conveying track is hindered, and a generation ratio of a paper jam and a wrinkle is increased. Especially in recent years, people are more environmentally aware, recycled paper is utilized of course, and since a using amount of pulp which is a raw material can be reduced, it is expected that a demand for thin paper sheets will increase more and more in the future.
The present invention provides an image forming apparatus in which a loop is formed in a sheet, but a sheet tip end does not collide against a mechanical sensor which detects a loop amount, appropriate loop control is performed without hindering a track when the sheet is conveyed, thereby suppressing generation of an image failure.